I see/read this term bandied about without any specific definition, as if everyone knows exactly what is implied in the context of its usage. However, I do not.

I've googled the net and searched these forums for an emphatic definition only to discover that mass loading is the classic definition of a horn in reverse (aka a tapered quarter wave), the addition of physical mass (sand, stone, concrete, et. al.) to an enclosure to reduce cabinet resonance, and/or "coupling" the rear of a driver (its magnet) physically to the cabinet with a brace (wood dowel or similar).

So, which one is being referred to when some one says, "You should mass load your design for better... blah, blah, blah" or "Your design is more like a mass loaded... blah, blah, blah"? That is, it seems to be used more frequently as a generic term in many cases of enclosure design and less often as a specific procedure.
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I think you have covered the possible uses for the term WRT speakers... there may be some others, but I suspect that most of the places where you hear this bandied about it is more like marketing hype than anything else... just speculating.

Although bracing a driver to the rear wall is not mass loading, afaik. Placing MASS (a weight) behind the driver might be called "mass loading" but that's not really "loading" exactly, imo.

It's more like the addition of weight (mass) to a Passive Radiator, or the cone of a woofer to lower the Fs, that is mass loading.

_-_-bear

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_-_-bear http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]

Some of those definitions sound like misunderstandings. And some do not apply to speakers-they apply to trying to reduce resonance in speaker stands or speaker enclosures. That's the most common use of the term "mass loading"-for instance, filling a hollow metal speaker stand with sand &/or lead shot to increase the mass and reduce vibration.

Applied to a speaker, it's different. The moving mass of a speaker consists of the physical moving parts PLUS the air load on the diaphragm. Mass loading thus means techniques which increase the air mass load.

For instance, sticking the speaker on a horn lets the speaker "grab" the nearby air more efficiently. Instead of trying to push all the air in the whole world, the diaphragm is just pushing air right in front of it and can push it more effectively. I suppose those analogies probably aren't technically correct, but useful for thinking.

Now the purpose of a horn is not really mass loading. But some makers put their subwoofer cones facing the floor, or firing into some kind of slot, or facing each other in a manifold. Again since the cone is not trying to grab all the air, but a concentrated piece of air right in front of it, the air mass will be somewhat higher.

From the hazy past, I don't recall if these techniques really change the total mass very much. My vague feeling is the sound is more likely affected by:
- Resonances in the cavity formed by the slots, which can introduce response peaks.
- Blockage of high frequencies from radiating directly to listener's ears, which may reduce the audible distortion.

"The ML TL enclosure can be thought of as a form of transmission line where quarter wavelength standing waves are used to provide the spring for the mass of air in the port. To physically model a straight uniform TL, clamp a yardstick to the edge of a counter or desk and pluck the free end so that it starts to vibrate. The vibration pattern is analogous to the air velocity in a TL. The TL's air velocity is zero at the closed end as is the yardstick's motion at the clamped end. The TL's air velocity is a maximum at the open end as is the yardstick's velocity at the free end.

There are two ways of changing the frequency of vibration for the yardstick. If you shorten the length cantilevered off the counter, the frequency of vibration will increase. Make the length longer and the frequency decreases. This is how straight TL's have traditionally been tuned by adjusting the length. The second way of tuning the frequency of the yardstick is to add a lump of mass to the free end. Put a piece of modeling clay on the free end and watch the frequency decrease. What I have done to the classic TL is put a lump of mass at the terminus end using a restrictive port. For a given frequency, I can shorten the TL (make it stiffer) increasing the tuning frequency and then add mass (air in a port) to pull the frequency back down and get a similar tuned result. One other benefit of having a lump of mass at the terminus is a rolled off port output above the first quarter wavelength resonance. This result is similar to a bass reflex port's response. I did this first with the ML TQWT and then with a straight TL. If you try the yardstick analogy, I think by changing the length and adding mass to the end you can demonstrate to yourself exactly what I am doing in my MathCad computer models."

Thank you! That was very helpful. I had a suspicion that the mass referred to had something to do with air volume/pressure/damping/etc. when coupled with QWL designs rather than the three definitions above. Your analogy of the vibrating ruler was superb.
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For instance, sticking the speaker on a horn lets the speaker "grab" the nearby air more efficiently.
snip

Right

Thoroughly stupid, when think about it, trying to vibrate the thin air with any kind of precision by shaking a massively heavy block of cardboard, even with a very strong motor (magnet).

Mass loading is crucial to good sound production just as shock absorbers are to a car and for analogous reasons.

(In reply to head unit, woofer horns do try to create air mass loading. And in such cases, they do increase efficiency through impedance matching - although efficiency per se tends not to be an important goal now-a-days.)

That is, it seems to be used more frequently as a generic term in many cases of enclosure design and less often as a specific procedure.

Hi Dean,

Mass loading usually means that the enclosure design includes a port or tube open at both ends that is short enough so that the air in the tube moves as a whole, without appreciable compression. In other words the component behaves primarily as an acoustic mass.

If a tube rigidly closed at one end is used instead, then the air will be compressed and the component acts primarily as an acoustic compliance.